This invention relates to a process for cooling a product in a heat exchanger employing microchannels for the flow of refrigerant and product through the heat exchanger. The process is suitable for liquefying natural gas.
Current commercial cryogenic processes for making liquefied natural gas (LNG) include the steps of compressing a refrigerant and flowing it through a spiral wound or brazed aluminum heat exchanger. In the heat exchanger the refrigerant exchanges heat with the natural gas and liquefies the natural gas. These heat exchangers are designed to provide very close temperature approaches between the refrigerant and natural gas streams that are exchanging heat. Increasing the thermal efficiency of these heat exchangers through changes in design or materials of construction typically results in increasing the capital cost of the heat exchanger, increasing the pressure drop for the refrigerant flowing through the heat exchanger, or both. Increasing the pressure drop results in increased compressor requirements. The compressor service required for these processes comprises a significant portion of the capital and operating cost of these processes. The problem therefor is to provide a process that results in a reduction in the pressure drop for the refrigerant flowing through the heat exchanger. This would improve the productivity and economics of the process. The present invention provides a solution to this problem.
Due to the large capital cost of cryogenic liquefaction, LNG plants are being built with ever-larger capacities in order to meet project economic targets through economies of scale. This need for economies of scale has resulted in increases in the size of single-train LNG processes. Currently, the size of a single-train LNG process with one compressor is limited by the maximum size of the compressors that are available. The problem therefor is to reduce the compressor requirements for these processes in order to increase the maximum size for the LNG process that is possible. This invention provides a solution to this problem.
Aluminum is typically used as a material of construction in conventional cryogenic heat exchangers. Aluminum minimizes heat transfer resistance due to the fact that it is a high thermal conductive material. However, since it is a high thermal conductive material aluminum tends to decrease the effectiveness of the heat exchangers due to axial conduction. This limits the ability to shorten the length of these heat exchangers and thereby reduce the overall pressure drop in them. An advantage of the present invention is that it is not necessary to use high thermal conductive materials such as aluminum in constructing the heat exchanger used with the inventive process.
This invention relates to a process for cooling a product in a heat exchanger, the process comprising: flowing a refrigerant through a set of first microchannels in the heat exchanger; flowing a refrigerant through a set of second microchannels in the heat exchanger, the refrigerant flowing through the set of second microchannels being at a lower temperature, a lower pressure or both a lower temperature and a lower pressure than the refrigerant flowing through the set of first microchannels; and flowing a product through a set of third microchannels in the heat exchanger, the product exiting the set of third microchannels having a cooler temperature than the product entering the set of third microchannels.
In one embodiment, the inventive process is operated using non-turbulent flow for the refrigerant flowing through the sets of first and/or second microchannels. Also, the microchannels may be relatively short. This provides for relatively low pressure drops as the refrigerant flows through the microchannels. These relatively low pressure drops reduce the power requirements for compressors used with such processes. For example, in one embodiment of the invention, a reduction in compression ratio of about 18% may be achieved for the inventive process used in making liquefied natural gas as compared to a comparable process not using microchannels for the flow of refrigerant in the heat exchanger.
Another advantage of the inventive process is that the use of microchannels in the heat exchanger decreases thermal diffusion lengths substantially as compared to prior art methods not using microchannels. This allows for substantially greater heat transfer per unit volume than is achieved with prior art heat exchange techniques.